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• Why does helium have the largest first ionisation energy?
• It's first electron is in the first shell, closest to the nucleus and has no shielding effects from inner shells. it has a bigger first ionisation energy than Hydrogen bcz it has 1 more proton.
• Why do first ionization energies decrease down a group?
• As you go down a group, outer electrons=found in shells further from the nucleus and are more shielded so the attraction of the nucleus becomes smaller.
• Why is there a general increase in first ionisation energy across a period?
• As you go across a period, the electrons are being added to the same shell